Development of fiber lasers has shown impressive progress during recent years. Early experiments on fiber lasers were done with active fibers with small core diameters capable of supporting only the fundamental optical mode. With such single mode active fibers output power levels of several hundreds of watts has been demonstrated, putting fiber lasers to the same or above the performance level of other lasers, such as solid state and gas lasers. Demonstrated power levels facilitate a number of materials processing applications for fiber lasers. Apart from continuous wave operation, pulsed operation and pulse amplification with down to femtosecond range pulse widths and peak power well in excess of 100 kW has also been demonstrated with fiber lasers. The preferred need to operate the lasers in fundamental mode leads to the requirement of using relatively small fiber core diameters when using conventional fiber designs, such as step index fibers. The small core area in such fibers, however, starts to limit the achievable power in both continuous wave (=CW) and pulsed fiber lasers and amplifiers. In CW fiber lasers, the upper limit of achievable power is set by the optical damage threshold of the fiber and stimulated Brillouin scattering (=SBS), while in the pulsed fiber lasers or amplifiers the peak power limit is often set by non-linear phenomena in the fiber, such as self-phase modulation and stimulated Raman scattering (=SRS). In order to avoid such power and/or pulse energy limiting mechanisms, one needs to increase the area of the optical field in the fiber. Efforts to do that by simply increasing the core diameter in a conventional step index fiber readily leads to multimode operation and reduced beam quality. The reason to this is that while increasing the core diameter one also must reduce the refractive index step between the core and the cladding materials to keep the fiber single moded. Eventually, manufacturing reproducibility and accuracy for the index step become a limiting factor for making a single-mode step-index fiber when the core diameter is increased beyond a couple of tens of microns. Other solutions to this problem have been developed, one of the most exciting being so called photonic crystal fiber (=PCF), where light confinement in the fiber core is established by making a periodic arrangement of air holes into the fiber that act as mirror. However, PCFs are known to have some drawbacks, namely, they are difficult to manufacture, they are quite sensitive to fiber bending, and also their splicing to conventional fibers is not easy without incurring losses.
Step-index fibers have almost exclusively been used for high power fiber lasers. In such fibers the active material of the core is surrounded by cladding material having a lower refractive index than that of the core. In the so called double-clad fibers an inner cladding material, typically a few hundred microns in diameter is subsequently surrounded by an outer cladding material having a refractive index lower than that of the inner cladding, and that confines the pump light into the inner cladding and makes optical pumping of the core feasible with light having a high numerical aperture. Various schemes have been used to couple the pump light into the active fiber, such as end coupling and side coupling through a pump fiber.
Step index fibers are characterized by a V-number, whose magnitude determines how many stable modes exist in the core of the fiber. V is given by V=πD/λ NA, where D is the diameter of the core, λ the wavelength, and NA the numeric aperture of the fiber, i.e. NA=√{square root over (n12−n22)}, where n1 is the refractive index of the core and n2 is the refractive index of the cladding. When V<2.4 the fiber supports only a single mode, and it is then called a single-mode fiber. From the definition of V it is easy to see that when the core diameter is increased it is necessary to have a smaller index step n1-n2 between the core and the cladding in order to keep the fiber single-moded. As an example, with a fiber having a core diameter of 30 μm, an index step of about 2×10−4 is needed for single-mode operation at 1 μm wavelength. On the other hand, reliable and reproducible manufacturing of fibers limit the minimum practical index step to a value of about 1×10−3. Hence, manufacturing tolerances for the index step become a limiting factor for making a single-mode step-index fiber when the core diameter is increased beyond a couple of tens of microns. Furthermore, the number of stable modes in the core, given roughly as 0.4×V2 for large values of V, increases quadratically as the core diameter D is increased. In large-core fibers there usually therefore exist many modes.
An active fiber used as a gain medium in a laser must not be truly single mode to achieve single mode operation of the laser. The lasing modal characteristics are not only determined by the passive fiber but rather by the net modal gain that depends on modal overlap With the active gain medium and modal losses due to e.g. modal leakage out of the core region. Both of these can be modified with fiber geometry, index profile, and bending or twisting of the fiber. Different fiber geometries, index and doping profiles have been proposed to increase the differential gain between LP01 and higher order modes. Some methods rely on changing the confinement of the active-ion doping profile in the fiber core for achieving the highest gain for the fundamental mode, as described in APPLIED PHYSICS LETTERS vol. 74, No. 11, 15 Mar. 1999: Sousa, Okhotnikov—“Multimode Er-doped fiber for single-transverse-mode amplification”, or alternatively adjusting the fiber index distributions of the core and cladding regions in order to keep the losses for the fundamental mode low enough as the NA of the core is reduced, as suggested in the U.S. Pat. No. 6,614,975. The problem of these methods is the manufacturability and reproducibility of the structures. The control of the doping profile and the location, width and refractive indices of the surrounding cladding layers is difficult. Alternative approaches exploit methods where significant losses are applied to all but the lowest-order modes. This can be done inducing significant bend loss for the higher order modes by coiling the fiber around a mandrel of suitable size, as described in the U.S. Pat. No. 6,496,301. The purpose of coiling the fiber is to induce significant radiative bend losses to the higher order modes i.e. to other than the fundamental mode. However, for large-core fibers this requires rather small bending radius that, on the other hand, may cause the fiber to break or adversely affects its durability. Alternatively, the loss for the higher order modes can be induced by manufacturing a secondary core of absorber material for absorbing radiation at unwanted modes, as disclosed in the U.S. Pat. No. 5,121,460. In summary, the operation of the coiled fiber amplifier/laser is limited by the manufacturing tolerances of the index step and the tight bending radius, which induces stress on the fiber. The latter approach is limited by the requirement of tight bending radii for large core sizes, which eventually reaches the limit of mechanical reliability or fracture of the fiber.
The U.S. Pat. No. 5,818,630 describes an optical amplification system, comprising: a laser source generating an input beam having a nearly diffraction limited mode; a multi-mode fiber amplifier; a mode converter receiving the input beam and converting the mode of the input beam to match a fundamental mode of the multi-mode fiber amplifier, and providing a mode-converted input beam to said multi-mode fiber amplifier; and a pump source coupled to said multi-mode filter amplifier, said pump optically pumping said multi-mode fiber amplifier, said multi-mode fiber amplifier providing at an output thereof an amplified beam substantially in the fundamental mode. Further in this optical amplification system the multi-mode fiber amplifier comprises a fiber core, wherein a dopant is confined in an area in a central section of the fiber core substantially smaller than a total fiber core area, and wherein mode-coupling into higher-order modes is reduced by gain-guiding. This kinds of systems have been disclosed in earlier publications like OPTICS LETTERS vol. 22, No. 6, Mar. 15, 1997: Taverner, Richardson, Dong, Caplen, Williams, Penty—“158-μJ pulses from a single-transverse-mode, large-mode-area erbium doped fiber amplifier”.
Publication WO-00/02290 describes an optical fibre having a cladding layer surrounding a core, the cladding layer comprising at least a first, relatively inner region, a third, relatively outer region, and a second region disposed between the first and third regions, the second region having a higher refractive index than the first and third regions; wherein the peak difference in refractive index between the first cladding region and the core is less than about 0.0030, or less than about 0.0025, or preferably less than about 0.0015. Accordingly, here is used the knowledge that the core diameter can be enlarged and hence the energy density therein can be reduced by lowering the refractive index difference between the energy transmitting core and the non-transmitting cladding. The core of the fibre according to the publication consists of a low numerical aperture central region that is doped with the active atoms and exhibit a refractive index dip, which central region is surrounded by an outer ring that is undoped with the active atoms and has a considerably higher refractive index than the central region. The fibre has a relatively large “multimode” core, which is operating in a single mode by the influence of the placement of the dopant, i.e. this property is achieved by the difference between the central region of the core and the outer ring of the core.